INDIANAPOLIS -- Special shoe releases from footwear companies such as Nike are fueling a massive resale market that is creating business opportunities for people who are plugged into "sneakerhead" culture.

Sneakers even have their own stock exchange — an online marketplace called StockX — where you can watch shoe values rise and fall.

Many old shoe styles are becoming more popular over time. It doesn't matter, for instance, that Michael Jordan retired in 2003. When the Air Jordan 2 Retro "Just Don" shoe went on sale in Chicago last year, people lined up around the block to pay $650 for a pair, according to The Chicago Tribune.

Some people might have put those retro Air Jordans on their feet. But others put them back up for sale as part of a secondary sneaker market that NPD analyst Matt Powell estimates is worth between $200 million and $500 million a year.

Paul Zigrang has tapped into that market — and found it to be even greater than he expected. Zigrang, 38, last year took the leap from selling shoes at once-a-month shows to opening a store called Grails at Washington Square Mall on Indianapolis' east side.

"We specialize in shoes (that are) already sold out everywhere else," Zigrang said. "It’s almost like a pawn shop/consignment shop where half the shoes in here are used, but it’s stuff that people really go after."

The inventory appeals to "normal people that miss out on limited shoes," he added.

The store performed so well that Zigrang opened an even bigger store a few months later.

Grails sells shoes that range in price between $25 and $2,000. Nike's Air Jordans and Adidas' Kanye West's Yeezy line are the most sought-after collections.

But the sneaker resale market has critics. Mike Watson, the owner of Got Sole, a 10-year-old boutique that's popular among sneakerheads, said he tries to keep collectors away from big releases.

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"We'll have limited edition shoes, but we're more in business to put shoes on people's feet," Watson said. "That's the double-edged sword when it comes to people who call themselves sneakerheads...Some people appreciate the design of the shoe, the story behind the shoe, and then there's people who could care less. They're just more interested in supply and demand."

Watson calls StockX "more hurtful than it is helpful" and considers himself "anti-resellers." The most popular shoes at Got Sole tend to sell for between $130 and $250, Watson said.

Zigrang said his business model can make shoes accessible to families that otherwise might not be able to buy high-end models.

"A lot of kids' parents can't afford $200 for shoes for their kids," he said. "So, if they have a shoe that they want to trade, (that will) lower the price. Then we also have shoes we sell for 40 percent off, 25 percent off. They're all really expensive (versions of) shoes — they're just gently worn."